1. Field of the Invention
This invention relates to a process for producing toner particles for rendering electrostatic latent images visible in image forming processes such as electrophotography, electrostatic recording, magnetic recording and toner jet recording.
2. Related Background Art
A number of methods are conventionally known as methods for electrophotography. In general, they are methods in which copied images are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, developing the latent image by the use of a toner to form a toner image, subsequently transferring the toner image to a transfer material such as paper as occasion calls, and then fixing the toner image thereto by the action of heat and/or pressure, solvent vapor or the like. As methods by which the electrostatic latent image is developed by the use of a toner or methods by which the toner image is fixed, various methods have been proposed, and methods have been employed which are suited for their corresponding image forming processes. In recent years, such electrophotographic processes are required to achieve higher-speed copying and higher image quality.
In general, the following processes are known as processes for producing toners. One of them is a process in which a colorant such as a dye or a pigment and additives such as a charge control agent are melted and mixed in a thermoplastic resin to effect uniform dispersion, followed by pulverization and classification by means of a fine grinding machine and a classifier to produce a toner having the desired particle diameter; i.e., a pulverization process.
In the production of a toner by such a pulverization process, there are restrictions where a releasing substance such as wax is added. More specifically, such restrictions are such that, in order to make the releasing substance have dispersibility at a satisfactory level, the releasing substance must keep its viscosity to a certain degree at the temperature at which it is kneaded with the resin, and that the releasing substance is kept in a content of about 5 parts by mass based on 100 parts by mass of the toner. Because of these restrictions, there is a limit to the fixing performance of the toner obtained by the pulverization process.
In such a pulverization process, it is also not easy to make fine solid particles such as the colorant perfectly uniformly dispersed in the resin, and the toner may come compositionally distributed depending on the degree of dispersion to cause variations of developing performance of the toner.
In the toner obtained by this pulverization process (hereinafter also “pulverization toner”), the step of classification is essential in order to attain stated particle diameter and particle size distribution, and, through this step, fine powder and coarse powder come in addition to the toner with stated particle diameter. Accordingly, various ideas are made on how to reuse them in production. The coarse powder is again pulverized in the production step to become finely pulverized. However, conventionally, the toner fine powder that has come has been reused by the recycling to the raw-material mixing step in its stated quantity from the viewpoint of environment and production cost (see, e.g., Japanese Patent Application Laid-open No. H05-34976). In this method, however, the molecules of the resin in the fine toner powder become again cut when the fine toner powder is again melt-kneaded with a kneading machine, to make the resin component have a low molecular weight. Hence, this causes hot offset or the like at the time of fixing of toner to paper to make fixing performance poor, undesirably.
In order to remedy such disadvantages, various ideas have been proposed in regard to the reuse of toner components, and the reuse of the fine toner powder by introducing it into the kneading step is in wide practice as a known technique from the viewpoint of how to produce toners well economically and in a good productivity (see, e.g., Japanese Patent Application Laid-open No. H08-69126). However, even in the recycling carried out by the above method or the like, the fine powder and coarse powder coming at the time of the classification step is commonly in a proportion of about 50 to 70% to the raw materials. Thus, the yield itself as a product is low, and is not preferable in view of production cost.
Against such a pulverization process, a toner production process is also proposed in which a polymerizable monomer composition having at least a polymerizable monomer is subjected to suspension polymerization to obtain toner particles simultaneously therewith (a suspension polymerization process). This suspension polymerization process is a production process as described below. First, a polymerizable monomer and a colorant (further optionally a polymerization initiator, a cross-linking agent and other additives) are uniformly dissolved or dispersed to obtain the polymerizable monomer composition. Then, this polymerizable monomer composition is dispersed in a continuous phase (e.g., an aqueous phase) containing a dispersion stabilizer, by means of a suitable stirrer to carry out polymerization reaction simultaneously to obtain toner particles having the desired particle diameter. This suspension polymerization process is free from the restrictions as stated on the above pulverization toner, and has various advantages.
More specifically, in regard to the content and dispersibility of a release agent (releasing substance), the toner obtained by the suspension polymerization process (hereinafter also “polymerization toner”) enables the release agent component to be enclosed in the toner particles, and hence its content can be made larger than that in the toner produced by the pulverization process. It can also satisfy the dispersibility simultaneously. Also, the dispersibility of the colorant does not particularly come into question because the colorant can uniformly be dissolved or dispersed in the polymerizable monomer together with other additives.
However, this polymerization toner as well has problems as stated below, which should be solved. In the suspension polymerization process, in the step of granulation which forms the toner particles, the polymerizable monomer composition is made into droplets and the toner particles come to be formed, in virtue of the shearing force produced by the rotation of stirring blades. It is preferable that the toner particles obtained here have a sharp particle size distribution. The feature that the toner particles have a sharp particle size distribution is an essential condition in order for the toner to exhibit a good developing performance. In general, in order to make the toner particles have a sharp particle size distribution, it is effective to make the stirring blades rotated in a high speed to impart a high shearing force. However, as the stirring blades are rotated in a higher speed, a vortex comes to be formed around the stirring blades, so that air bubbles begin to be incorporated into the materials being treated. Hence, the loss of shearing force comes about in proportion to the speed made higher. In order to prevent this, it is effective to provide a baffle or the like, which, however, is not sufficiently effective. Also, as the stirring blades are rotated in a higher speed, cavitation begins to take place around the stirring blades. Hence, at a certain number or more of revolutions, the particle size may no longer become sharper to make the particle size distribution poor. Thus, although it is an effective means to make the stirring blades rotated in a higher speed in order to make the toner particles have a sharper particle size distribution, such a means may even bring about a reverse effect at a certain number or more of revolutions. Hence, there is a limit in making the particle size distribution sharp.
Meanwhile, depending on the concentration of a dispersant used in the aqueous phase in order to provide a sharp particle size distribution width and on the conditions under which it is added, polymerization may concurrently occur in the aqueous phase to form ultrafine particles of 0.1 to 1 μm in particle diameter or smaller in size than that. Such ultrafine particles make non-uniform the dispersibility of the colorant and so forth in those particles, and the presence of the ultrafine particles makes the toner cause problems on image characteristics (solid density, density uniformity, fog and so forth). Further, once such ultrafine particles have adhered to toner particle surfaces, the toner may change in its fluidity and charge controllability, and hence this likewise makes the toner cause problems on image characteristics.
With a trend toward higher image quality in electrophotography, the particle size distribution required in the polymerization toner is also required to be made much sharper. However, in existing techniques, however the conditions for granulation are optimized, it may become necessary to lessen the proportion of fine particles of 4 μm or less in particle diameter and coarse particles of 10 μm or more in particle diameter through the step of classification.
From another point of view, in the polymerization toner, particles are also commonly so designed as to have a core-shell structure having at least two layers, in which the releasing component, low-energy fixing component and so forth stand enclosed in toner particles. Hence, even where toner particles outside the range of stated particle size distribution or particle size distribution width have been formed in any form, they can not be reused in such a simple way as in the pulverization toner. Thus, the fine particles and coarse particles formed as by-products at the time of the classification step have caused a rise in cost.